1. Field of the Invention
Aspects of the present invention relate to a direct liquid feed fuel cell system having a compact air supply line and a fuel cell stack cooling device.
2. Description of the Related Art
A direct liquid feed fuel cell is an apparatus that generates electricity through an electrochemical reaction between an organic fuel, such as methanol or ethanol, and an oxidant, i.e., oxygen. The electricity generated by a direct liquid feed fuel cell has high specific energy density and high current density. Also, since a liquid fuel, e.g., methanol, is fed directly to an anode electrode, the direct liquid feed fuel cell does not require a peripheral device, such as a fuel reformer, and the liquid fuel is stored and supplied easily.
FIG. 1 is a cross-sectional view of a direct liquid feed fuel cell. Referring to FIG. 1, the direct feed fuel cell has a structure in which an electrolyte membrane 1 is interposed between an anode 2 and a cathode 3. The anode 2 includes a diffusion layer 22 to supply and diffuse a fuel, a catalyst layer 21 where an oxidation reaction of the fuel occurs, and an electrode supporting layer 23. The cathode 3 also includes a diffusion layer 32 for supplying and diffusing an oxidant, a catalyst layer 31 where a reduction reaction of the fuel occurs, and an electrode supporting layer 33. The catalyst of the electrode reaction is formed of a noble metal having superior catalytic characteristics at low temperatures, such as platinum, and to avoid catalyst poisoning by carbon monoxide (CO), which is a by-product of the electrode reaction, the catalyst can be an alloy of a transition metal, such as ruthenium, rhodium, osmium, or nickel. The electrode supporting layers 23 and 33 can be made of waterproof carbon paper or waterproof carbon fiber to easily supply fuel and discharge reaction products. The electrolyte membrane 1 is a hydrogen ion exchange membrane that has an ion conductivity and can contain moisture, and may be a polymer membrane having a thickness of 50-200 μm.
An electrode reaction of a direct methanol fuel cell (DMFC), which is a type of direct liquid feed fuel cell, includes an anode reaction where fuel is oxidized and a cathode reaction where hydrogen and oxygen are reduced, as described below.CH3OH+H2O→CO2+6H++6e−(Anode reaction)  [Reaction 1]3/2O2+6H++6e−→3H2O(Cathode reaction)  [Reaction 2]CH3OH+3/2O2→2H2O+CO2(Overall reaction)  [Reaction 3]
Carbon dioxide, hydrogen ions, and electrons are produced at the anode 2 where the fuel is oxidized (reaction 1). The hydrogen ions migrate to the cathode electrode 3 through the electrolyte membrane 1. Water is produced by the reduction reaction between the hydrogen ions, electrons transferred from an external circuit, and oxygen at the cathode 3 (reaction 2). Accordingly, water and carbon dioxide are produced as the result of the overall electrochemical reaction (reaction 3) between methanol and oxygen. Two moles of water are produced when one mole of methanol reacts with oxygen.
The liquid fuel used in the fuel cell may be a mixture of pure methanol and water produced in the system or already stored in the fuel cell. When a fuel of high concentration is used, the performance of the fuel cell is greatly reduced due to crossover of the fuel at the electrolyte membrane (hydrogen ion exchange membrane). Therefore, methanol diluted to a low concentration, such as 0.5 to 2 M (2 to 8 volume %), is generally used.
The theoretical voltage that can be generated by a unit cell of a DMFC is approximately 1.2 V. However, an open circuit voltage at an ambient temperature and at atmospheric pressure is less than 1 V due to a voltage drop caused by an active over-voltage and a resistance over-voltage. In reality, an actual operating voltage lies in the range of 0.4-0.6 V. Therefore, to obtain higher voltages, a plurality of unit cells is connected in series. Unit cells can be connected to form a monopolar structure in which a plurality of unit cells are arranged on one electrolyte membrane, or a stack structure in which a plurality of unit cells are stacked.
A fuel cell can be used as a power source of a mobile electronic device, for example, a computer notebook. A fuel cell used for a mobile electronic device requires a compact structure. On the other hand, fuel cells generate a lot of heat due to the electrode reactions, and in some cases, a fuel cell temperature may reach 70° C. However, to use fuel cells in electronic devices, generally, the fuel cell temperature is required to be maintained below 60° C.